Molded workpieces of any three-dimensional form, including fibers and filaments, and films based on polyolefins, more particularly polyethylene and polypropylene, are used on a very wide scale in practice. An important problem area here is improving the surface properties of these structurally nonpolar hydrocarbon components. Thus, the inadequate adhesion of coatings, adhesives, printing inks and the like is a central problem which has remained unsolved for decades despite numerous attempts to find a solution.
A comprehensive account of the various proposals which have been made with a view to solving the stated problem can be found, for example, in EP 0 311 723. In the interests of simplicity, reference is hereby made to the disclosure of this document.
The concept of improving adhesion to hard polyolefin surfaces in reality encompasses an extremely broad range of very different technical requirements. This will immediately become clear from the following comparison: improving the adhesion of a printing ink to polyolefin films cannot be compared with improving the adhesive bonding to a rigid polyolefin molding of solid workpieces of metal or other plastics with no risk of adhesive failure when mechanical forces are applied to the bond. Of considerable practical significance moreover is the dependence on time of the adhesive strength established, the notion of dependence on time in turn encompassing two different parameters. The first concerns the time interval elapsing between production of the polyolefin-based molding and its subsequent coating or bonding. The second quality parameter concerns maintenance of the adhesiveness values initially established in the bonded workpiece or coated material over the duration of its practical use which, as well known, can involve very long periods of time, for example of several years. What is required in practice for a number of applications of the technology under discussion here is the possibility of establishing high-strength bonds by coating and/or application of adhesives using standard auxiliaries up to and including resistance to adhesive failure with no dependence on time between production and finishing of the polyolefin-based molding on the one hand and its coating and/or bonding on the other hand, the strength values once established remaining intact for virtually indefinite periods.
The teaching according to the invention starts out from this objective. In addition, the invention seeks to create the possibility of not necessarily having to use past technical proposals for solving the problem discussed in the foregoing. Thus, it is known that the adhesive strength of polyolefin-based materials can be substantially increased by processing high molecular weight copolymers containing coupling polar groups in the polymer structure together with the nonpolar polyolefin and/or by imparting the permanent high polarity required to the nonpolar polyolefin by grafting on polar elements, cf. for example the disclosures of DE 34 90 656, EP 0 467 178 and JP 222 181.
Numerous proposals from the prior art are also concerned with incorporating comparatively low molecular weight mixture components containing polar substituents in the polyolefin-based polymer. The proposals in question are mainly concerned with improving the adhesion of, above all, printing inks to polyolefin films, i.e. with fulfilling a relatively moderate technical requirement. The following documents are cited purely by way of example as representative of the extensive prior-art literature:
DE-OS 27 29 886, EP 0 402 100, U.S. Pat. Nos. 3,396,137, 5,286,525 and 5,393,814 and JA 0 54 331. The cited documents are mainly concerned with improving the adhesive strength of polyolefin films with respect to the application of printing inks. The use of known surface treatments, more particularly corona or plasma pretreatment, is also prescribed.
DD-PS 50 947 is concerned in its introduction with proposals to incorporate low molecular weight additives containing polar substituents in polyolefin-based molding compositions. Reference is made here to the above-mentioned fact that, where low molecular weight internal additives such as these are incorporated, the corresponding polyolefin moldings show decreasing strength values and hence deteriorations in their mechanical properties after prolonged exposure to mechanical forces.
The reason for this unwanted development of the property spectrum of correspondingly finished materials lies in the known migration of low molecular weight components containing polar substituents in admixture with the polyolefin. Even if these low molecular weight additives are uniformly incorporated, the effect known as blooming-out occurs. During storage of the polymeric molding containing low molecular weight components or the corresponding film, the low molecular weight additives migrate outwards to the surfaces of the molding. Ultimately, even the hydrocarbon residues of the additives separate from their physical mixture with the polyolefin substance. The additive then exists as a liquid or solid film on the polyolefin surface. This naturally has a critical influence on the increasing deterioration in the adhesion values in the corresponding composite material. For comparatively slight stressing, for example the printing of polyolefin films, the immediate result does not have to be technical uselessness, especially since other properties of the polyolefin surface, for example slip, can be improved by blooming out. As mentioned, however, this phenomenon of migration and blooming out is unacceptable for the establishment of high-strength bonds and/or coatings with foreign materials without any dependence on time.